In a hydrostatic transmission cold starting difficulties can occur if the hydraulic medium flowing in the hydraulic circuit of the hydrostatic transmission undercools and therefore becomes viscous. These difficulties increase with increasing distance between the pump and hydraulic motor, i.e. with increasing length of the main line in which the pump and the hydraulic motor are arranged.
With increasing length of the main line a further problem arises in the return of leakage from the hydraulic motor to the tank. To avoid a long leakage or drain line extending from the hydraulic motor up to the neighbourhood of the pump it has already been proposed to connect the leakage or drain line of the hydraulic motor to the low pressure section of the main line. In such an arrangement overloading and operating difficulties of the hydraulic motor are to be expected if an increase in pressure occurs in the low pressure line, which can likewise be caused by cold, e.g. by at least local "freezing" of the low pressure line, since the viscosity of the operating fluid, the delivery flow and the line resistance determine the pressure in the return line and thus also the pressure in the housing of the hydraulic motor. For constructional reasons, and having regard to the seals of the hydraulic motor, e.g. a shaft seal, the maximum permissible pressure head is very limited. In order to prevent excessively high pressures the viscosity values of the operating fluid must be selected in relation to the operating and ambient temperatures that occur. In the case of extremely low temperatures and in the case of high line resistances this is in most cases not enough and, in particular in starting processes, the operating fluid has to be heated up accordingly before operation at nominal power is possible. It would, e.g. be possible to heat up the operating fluid before starting by means of a separate heating device.
A by-pass circuit has also been proposed comprising a valve chain arranged in a bypass connecting the high pressure line and the low pressure line to one another, having a pressure relief valve that is set so that, having regard to the maximum permissible return pressure, only a fraction of the normal delivery flow circulates in the main line. The circuit can be heated up by the heat arising from energy losses at the pressure relief valve.
These means are disadvantageous for the following reasons: Generally, only the existing transmission container for the hydraulic fluid is suitable for the preheating. Hence the hydraulic fluid in the main line is not included in the heating and therefore there is the danger that when starting the pressure in the low pressure line, and thus also the pressure in the housing of the hydraulic motor, may rise to an excessively high value at which damage to the hydraulic motor may occur.